Water heating devices are applied in all kind of applications, such as steam irons, electric kettles, hot drink vending machines, etc. A problem with such devices is that scale may form on the heating elements that are in contact with the water.
During operation of for instance a steam generation device, water is supplied to a part of the water infrastructure where it is heated, such as in the (external) boiler of a system iron, as a consequence of which scale may be formed. If the scale is not (periodically) removed clogging up may occur, as a result of which the performance of the steam generation device may decrease and, eventually, the steam generation device may not be fit to be used anymore.
WO2007007241 for instance describes a steam ironing system comprising a steam iron and a boiler system having a boiler for generating steam, wherein the steam iron and the boiler are connected to each other through a steam hose. During operation of the boiler system, scale is formed in the boiler. For the purpose of removing the scale from the boiler, a rinsing process is performed on the boiler system at regular intervals. During the rinsing process, a rinse valve connected to a water outlet positioned at a bottom of the boiler is opened, and water is discharged from the boiler to a rinse container. In the process, scale particles are carried along with the flow of water. Preferably, pressure is built up inside the boiler prior to opening the rinse valve, so that the water is forcibly ejected from the boiler, whereby the effectiveness of the rinsing process is enhanced. During the rinsing process or at the end of the rinsing process, scale solvent may be introduced into the boiler.
US 2003192436 describes a method for the generation of steam, in particular for a cooking device, whereby a fluid within a steam generator container is brought to the boil by the heating of at least one heatable wall surface of the steam generator container. Said fluid is set in rotation by the heating and thus pressed against the heatable wall surface, due to centrifugal forces arising from the rotation. The steam generated as a result of the evaporation of at least a part of the fluid escapes from the steam generator container through a steam outlet and droplets of fluid entrained in the steam are separated out by at least one first rotor, rotatably mounted in the steam generator container, whereby the fluid is set in rotation by said rotor. The invention further relates to a device using the above method.
US2001018866 describes an arrangement for indicating the calcification status of flow heaters, particularly in espresso machines, with the flow heater having a cold-water inlet line and a hot-water outflow line, and which is intended to precisely indicate the calcification status, yet have an uncomplicated design. For this purpose, a differential-pressure fluid gauge is provided, with the gauge having two pressure chambers separated by a diaphragm, with one of the pressure chambers being in a fluid-conducting connection with the cold-water inlet line and the other pressure chamber being in a fluid-conducting connection with the hot-water outflow. The diaphragm acted upon by the pressure difference between the two chambers is coupled to indicator elements.
Hard water containing significant amount of Ca2+ and HCO3− (bicarbonate) may form scale (CaCO3) upon temperature increase via the following chemical reaction:Ca(HCO3)2→CaCO3+H2O+CO2 
Especially boiling water will separate scale, the scale will form in the water but also on the heating element itself as it has the highest temperature. In time scale will grow on the heating element and when internal stresses increase it will break loose from the element. Several treatments of water to prevent scaling have been claimed in the literature. A well known method is the use of ion exchangers were Ca2+ is exchanged for Na+ or H+. A second well known method is the use of phosphonate that in small amount is added to the water and inhibits the formation of seeding crystals in the hard water, effectively preventing the growing of crystals and thus the formation of scale (see also the cited art above).
In the former a cartridge needs to be used with the ion exchange resin inside. After depletion the cartridge needs to be regenerated or replaced with a new one. In the latter case continuously phosphonate needs to be added as the phosphonates have limited stability at pH 7-8.5, the pH of hard water. The continuous addition can be implemented for example by using a hard-pressed tablet that very slowly liberates the phosphonates into the water. This way of working has been used in prior art steam irons. However, chemicals are added into the water which may be a drawback, for instance when water is (also) meant to be potable.
Physical methods to prevent scale formation have also been claimed but these may have a less clear working principle and the efficacy may sometimes even be doubtful in some cases. For example the use of (electro) magnets placed on water tubing for scale prevention is an example of a poorly understood and non-repeatable scale prevention method.